- Email: [email protected]
water mixtures
Journal of Membrane Science 209 (2002) 493–508
Pervaporation with chitosan membranes: separation of dimethyl carbonate/methanol/water mixtures Wooyoung Won a , Xianshe Feng a,∗ , Darren Lawless b a
Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 b Fielding Chemical Technologies Inc., Mississauga, Ont., Canada L5C 1T7 Received 15 February 2002; received in revised form 20 May 2002; accepted 29 May 2002
Abstract This study deals with the separation of binary dimethyl carbonate (DMC)/methanol, DMC/water, and methanol/water mixtures as well as ternary DMC/methanol/water mixtures by pervaporation using chitosan membranes. It is relevant to the manufacturing of DMC, where the energy intensive extractive distillation or pressure swing distillation is used conventionally for the separation of the reaction mixtures. Chitosan membranes were prepared by solution casting, followed by alkaline treatment. The effects of feed composition and operating temperature on the separation performance were investigated, and the membrane properties under the experimental conditions that are of interest to the manufacturing of DMC were evaluated. It was demonstrated that the membrane exhibited good performance for the DMC/methanol separation as well as the dehydration of DMC. The membrane also showed the capability of dehydrating methanol, but with a lower permselectivity. For the separation of ternary DMC/methanol/water mixtures, the interactions among the permeating components were shown to have a significant effect on the membrane performance. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Chitosan membrane; Dimethyl carbonate; Methanol
1. Introduction Dimethyl carbonate (DMC) is an important chemical that is considered to be an environmentally benign building block. The application of dimethyl carbonate has been expanding rapidly in recent years. It has been used as a carbonylation agent to substitute the deadly phosgene for the manufacturing of polycarbonates and urethane polymers, and as an environmentally friendly substitute to replace dimethyl sulfate and methyl halides in methylation reactions [1]. Numerous studies have been done to evaluate the advantage ∗ Corresponding author. Fax: +1-519-746-4979. E-mail address: [email protected] (X. Feng).
of using dimethyl carbonate for other applications including, for example, as a low viscosity solvent in lithium ion battery, and as an oxygenate for internal combustion engine fuels. The Federal Reformulated Fuels Act of 2000 requires the US EPA to ban the use of methyl tert-butyl ether (MTBE) in gasoline due to environmental concerns [2]. Consequently, dimethyl carbonate has become a strong contender for replacing MTBE as an oxygenate in the unleaded gasoline to meet the Clean Air Act specifications of oxygen-laden additives in gasoline, and it is anticipated that the production of DMC will be increased substantially to meet the refinery industry’s potential need. Dimethyl carbonate has excellent characteristics as a fuel additive, including
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 3 6 7 - 8
494
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
high oxygen content (53 wt.%), good blending properties, favorable distribution in gasoline/water, low haze point, low toxicity, and fast biodegradation. Though DMC has not been used in gasoline commercially, the idea of using DMC in fuels is not new. In 1943, it was reported that DMC and other alkyl carbonates could reduce the surface tension of diesel fuels [3]. The beneficial fuel properties of DMC were disclosed in several patents in the 1980s; a chronological development on this account is described by Pacheco and Marshall [4], who also predicted that a substantially large scale-up of current DMC production would be needed to meet the potential market need. Separation is an important step in the DMC manufacturing. For example, in the ENIChem process, which is a major commercial process for DMC production based on oxidative carbonylation of methanol on a CuCl catalyst, the product stream comprises of a ternary DMC/methanol/water mixture. Removal of methanol, water or methanol/water from the mixture is especially important due to the azeotropic nature of the mixtures. Various separation processes (e.g. extractive distillation, pressure swing distillation, liquid–liquid extraction, adsorption on zeolites, and low temperature crystallization) have been proposed to separate and purify DMC [4]. Pervaporation separation, which is based on the selective permeation of components in a mixture through a membrane, appears to be a promising alternative. The idea of using pervaporation for separation of DMC/methanol mixtures was disclosed in a few patents [5–8]. Shah et al. [9] compared high-pressure distillation with a hybrid membrane/distillation system and concluded that the hybrid system could save both capital and operating costs. In the hybrid system, the pervaporation membrane unit removes methanol from the reactor effluent prior to distillation in order to suppress the formation of azeotrope. Rautenbach and Vier [10] carried out a simulation study for DMC/methanol separation with a hybrid membrane/distillation system; the feed was admitted to the distillation column or the pervaporation unit if it was at the azeotropic composition. It was shown that the hybrid system compares with the conventional two-pressure distillation favorably. In spite of the interest in separating DMC using pervaporation, very few studies are available in the
Table 1 Membranes used for methanol/MTBE separation by pervaporation Membrane
Reference
Chitosan Polyvinyl alcohol Cellulose esters Poly(phenylene oxide) Polypyrrole Polyelectrolyte Agarose Acrylonitrile copolymer
[16,18,20,26] [12,22,23] [11,14,15,21] [24] [25] [13] [17] [19]
literature that address the preparation and performance of pervaporation membranes for DMC separation. This is presumably due to the fact that MTBE has been an authorized oxygenate in gasoline since the late 1970s and that MTBE have been the dominating cost-effective oxygenate ever since. In fact, the MTBE/methanol separation, which is relevant to MTBE manufacturing, by pervaporation has been investigated extensively, as illustrated in Table 1, which summarizes the recent work on membranes used for this separation. Considering the physicochemical similarities of MTBE and DMC, the prior work on pervaporation separation of MTBE/methanol mixtures suggests that chitosan- and polyvinyl alcohol-based polymers may be good candidate materials for preparing the pervaporation membranes for DMC/methanol separation. In the present study, the pervaporation separation of binary DMC/methanol, DMC/water, methanol/water mixtures and ternary DMC/methanol/water mixtures by chitosan membranes were studied. Chitosan is a polysaccharide that is insoluble in water at neutral pH’s. It is a linear polymer comprised primarily of glucosamine [27]. Chitosan was chosen as the membrane material because of its good film forming property and chemical stability as well as its demonstrated favorable performance for MTBE/methanol separation. The membranes were prepared by casting aqueous protonated chitosan solution, followed by alkaline treatment to convert chitosan in its free amine form. The effects of feed composition and temperature on the membrane performance were investigated, and the membrane properties under the experimental conditions that are of interest to the manufacturing of DMC were evaluated.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
495
2. Experimental 2.1. Materials Chitosan (Flonac N, molecular weight 100,000) in flake form was supplied by Kyowa Technos Co. Ltd., Japan. Methanol, ethanol and acetic acid were purchased from BDH Chemical, and dimethyl carbonate was purchased from Aldrich Chemical Company. All the organic solvents used in the experiments were of regent grade. De-ionized laboratory water was used in making the aqueous solutions for membrane preparation and treatment. The feed mixtures used for the pervaporation experiments were prepared by blending methanol, dimethyl carbonate and/or water with a predetermined composition. 2.2. Membrane preparation Homogeneous dense chitosan membranes were prepared by the solution casting technique. Firstly, chitosan was dissolved in a dilute aqueous acetic acid solution where the amine groups of chitosan were protonated by the acid to form a homogeneous solution. The polymer solution, comprising of 1.1 wt.% chitosan, 2 wt.% acetic acid, and 96.9 wt.% water, was filtered to remove a trace amount of non-dissolved residual solids. The chitosan solution was then cast onto a horizontally positioned glass plate with a Gardner knife at ambient conditions. The cast chitosan solution film was consequently air-dried in an environmentally-controlled chamber, which was supplied by D.F.S. Inc., France. The air-dried chitosan membrane, which was in the form of chitosan salt, was subject to an alkaline treatment so that the cationic amine groups (–NH3 + ) in chitosan were converted into the free amine form (–NH2 ). The alkaline solution contained 0.8 M sodium hydroxide in a 50/50 (v/v) ethanol/water solution. After alkaline treatment for 24 h, the membrane was thoroughly washed and rinsed with de-ionized water, and finally dried in ambient air. The thickness of the resulting membrane was 28.4 m. 2.3. Pervaporation The apparatus used for pervaporation experiments is shown schematically in Fig. 1. The membrane was
Fig. 1. Schematic diagram of pervaporation setup: (1) thermostat control; (2) feed tank; (3) heating device; (4) feed circulation pump; (5) pervaporation cell; (6) cold trap; (7) vacuum pump.
housed in a pervaporation cell that consisted of two detachable stainless steel parts. A porous steel plate was imbedded in one of the parts to support the membrane, and the two parts were set in proper alignment. Rubber O-rings were used to provide a pressure tight seal between the membrane and the permeation cell. The effective area of permeation was 14.2 cm2 . The feed mixture with a predetermined composition was circulated from a thermostated feed tank to the permeation cell using a feed pump, and the retentate from the permeation cell was recycled to the feed tank. Vacuum was applied to the permeate side of the membrane, and the permeate vapor was condensed and collected in a Pyrex glass cold trap immersed in liquid nitrogen. In all the experiments, the feed was kept at atmospheric pressure, whereas the permeate pressure was maintained at 2–3 mbar, which was monitored by a Pirani vacuum gauge (MKS Instruments). The permeation rate was determined gravimetrically
496
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
by weighing the permeate sample collected over a given period of time using an analytical balance, and the permeate composition was analyzed using a Hewlett-Packard gas chromatography (HP5890 Series II) equipped with a thermal conductivity detector. The feed circulation rate was kept substantially high as compared to the permeation rate so as to minimize the effect of concentration polarization. Though the pervaporation was carried out batchwise, the amount of permeate removed by the membrane was kept below 0.1% of the initial feed loading so that the feed concentration was essentially constant during pervaporation runs. This study was concerned
with steady state permeation. All the experimental data were collected after steady state of pervaporation had been reached. The time required to reach the steady state was between 1 and 4 h, depending on the permeation flux through the membrane. The membrane was tested for the separations of binary DMC/methanol, DMC/water and methanol/water mixtures as well as ternary DMC/methanol/water mixtures. The permeate concentration and the permeation flux were used to characterize the membrane performance. In some cases involving the permeation of binary mixtures, separation factor was also used to characterize the permselectivity of the membrane.
Fig. 2. Total permeation flux and water concentration in permeate for separation of water from dimethyl carbonate.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
3. Results and discussions Dimethyl carbonate and water are partially miscible. At 20 ◦ C, the aqueous DMC solution undergoes a phase separation when the water content is in the range of 2.7–87 wt.% [28]. Typically, the product stream from a DMC reactor, consisting of 50–70% methanol, 30–40% DMC and 2–5% water, is subject to distillation where the DMC/methanol azeotrope from the top of the distillation tower is recycled back to the reactor, whereas the water “wet” DMC is withdrawn from the bottom. The wet DMC is then cooled to induce phase separation, and both
497
phases can be further distilled until the DMC/water azeotropic composition is approached. The DMC product so obtained may still contain a small amount of water. Obviously, from a separation point of view, it is of interest to investigate pervaporation separation of the binary and ternary mixtures, especially when (i) the composition is close to the azeotropic point, or (ii) water is the minor impurity in the DMC product that needs to be removed. This study was primarily focused on dehydrating DMC, de-bottlenecking DMC/methanol azeotrope, and the removal of water/methanol from ternary DMC/methanol/water mixtures.
Fig. 3. Partial permeation fluxes of dimethyl carbonate and water for the dehydration of dimethyl carbonate.
498
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
3.1. Pervaporation of binary DMC/water mixtures The concentration range for homogeneous binary DMC/water solution is narrow, and caution was exercised in the measurement of permeate composition in case there was a phase separation in the permeate. Three concentration levels were chosen for the DMC dehydration study: 97.4, 98, and 99% of DMC by weight. The operating temperature was varied from 25 to 65 ◦ C for each concentration level. Over this experimental range explored, the permeate water concentration was found to be over 95 wt.%, within the homogeneous region of the DMC/water mixtures. Fig. 2 shows the total permeation flux and the perme-
ate concentration at different feed concentrations and operating temperatures. It can be seen that the water concentration in the permeate tends to be higher at a higher content of feed water. At a water content of 2.6% in feed, the permeate water content of greater than 97.5% was obtained in the temperature range tested, which corresponds to a separation factor of approximately 1460. Similarly, at a feed water content of 2%, the permeate water concentration was shown to be over 95%, corresponding to a separation factor of 930. Over the entire temperature range tested, the concentration of the permeate did not change significantly, which implies that the membrane is thermally stable for this system. The data in Fig. 2 also showed that the
Fig. 4. Pervaporation performance for the separation of binary dimethyl carbonate/methanol mixtures at different feed concentrations and operating temperatures.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
higher the feed water content is, the more significantly the permeation flux is affected by the temperature. Nickel et al. [7] reported that at a water content of 2.7% in the feed, a permeation flux of 0.8 kg/(m2 h) and a separation factor of 644 were obtained with the polyvinyl alcohol/polyacrylonitrile composite membrane developed by GFT at a temperature that was presumably between 50 and 70 ◦ C. Apparently, the chitosan membrane studied here is more selective to the DMC/water permeation, and the relatively low flux
499
of the membrane is due to the fact that the membrane used for this study was a thick dense homogeneous membrane. The permeation flux can be increased by the use of asymmetric composite membranes where the effective thickness of the membrane is reduced. It is interesting to note that unlike many other pervaporation systems where an increase in temperature generally leads to an increase in the permeation flux, the chitosan membrane showed a negative temperature dependence of the total permeation flux.
Fig. 5. Partial permeation fluxes of dimethyl carbonate and methanol for DMC/methanol separations.
500
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
This can be explained in the following. Based on the solution–diffusion model, the sorption and diffusion are the two major steps in pervaporation transport that control the permeation. In many cases, the diffusion is the rate-controlling step, and the effect of diffusion on the total flux is more significant than that of preferential sorption. The characteristics that distinguishes the permeation behavior of the DMC/water system from others is the poor miscibility of water with DMC. As the temperature increases, the solubility of water in DMC increases. As a result, at a given concentration, the activity of water in the homogeneous DMC/water solution decreases since more water can be dissolved by the DMC. In other words, the forces that trap water in the solution become stronger, which results in less water being sorbed into the membrane matrix, thereby slowing down the diffusion rate of water because of the reduced driving force across the membrane. In addition, the reduced sorption of water in the membrane matrix due to temperature increase will also reduce the plasticization of the membrane, and consequently the membrane displays a stronger resistance
to diffusion of both components. This hypothesis is in agreement with the experimental observations that increasing the temperature reduces both the partial fluxes of DMC and water, as shown in Fig. 3, which also implies that the plasticization of the membrane by water significantly affects the DMC permeation. At a higher water content in the feed, the permeation flux of DMC tends to be higher. It may be pointed out that DMC and water form an azeotrope at 77.5 ◦ C with a composition of 11% water and 89% DMC [29]. As such, from a practical application point of view, such an azeotrope can easily be broken by simply lowering the temperature to induce phase separation. Therefore, the pervaporation separation was not targeted to this azeotrope mixture in the present study. On the other hand, the membrane preferentially permeates water, and it is not anticipated to be effective to preferentially remove small amounts of DMC dissolved in water where water is the major component. A hydrophobic membrane that preferentially permeates DMC would be preferred for such an application.
Fig. 6. Permeate concentration as a function of feed concentration for pervaporation separation as compared to the vapor–liquid equilibrium (VLE) data.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
3.2. Pervaporation of binary DMC/methanol mixtures Fig. 4 shows the performance of the chitosan membrane for the separation of binary DMC/methanol mixtures at different operating temperatures. The membrane is preferentially permeable to methanol, and at a given temperature, as the methanol content in the feed increases, the overall permeation flux increases whereas the separation factor decreases. Unlike the aforementioned water/DMC separation, an increase in the operating temperature increases the overall permeation rate, and the separation factor, however, tends to decrease. Similar results have been observed for other pervaporation systems that
501
separate the MTBE/methanol mixtures [22,25,26]. A closer look at the partial fluxes, which are shown in Fig. 5, shows that both partial fluxes of DMC and methanol increase as the temperature increases, but the increase in the partial flux of DMC is generally more significant than that of methanol, leading to a gradual decrease in the separation factor. It may be pointed out that at a given operating temperature, the partial flux of methanol, which permeates through the membrane preferentially over DMC, tends to increase as the feed methanol concentration increases, whereas no such an obvious trend can be observed for the DMC permeation. It appears that at relatively low DMC concentrations, the DMC flux increases as
Fig. 7. Total permeation flux and separation factor for methanol dehydration at different operating temperatures and feed concentrations.
502
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
the DMC concentration increases and the opposite holds when the DMC concentration is sufficiently high. The experimental data of Nickel et al. [7] for DMC/methanol separation using a GFT membrane showed similar trend. Pasternak et al. [8] tested polyvinyl alcohol membranes for the separation of a methanol/DMC mixture that was close to the azeotropic composition (comprising of approximately 70/30 methanol/DMC by weight), and a permeate concentration of 93–97 wt.%
methanol and a total flux of 100–1130 g/(m2 h) were obtained, depending on the curing conditions of the membrane. This is comparable to the results obtained in this study. Fig. 6 shows the permeate concentration as a function of feed concentration as compared to the vapor–liquid equilibrium (VLE) data; for comparison, the pervaporation data of Rautenbach and Vier [10] using the GFT membrane at 70 ◦ C are also shown. The chitosan membrane appears to be more selective than the GFT membrane, which was
Fig. 8. Illustration of the effect of feed composition on the pervaporation performance of methanol/water separation at 45 ◦ C.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
primarily developed for dehydration of alcohols. It is clear that the concentration of methanol in the permeate is much higher than that in the saturated vapor over the entire concentration range. These results indicate that pervaporation is more efficient for DMC/methanol separation than distillation, and that the azeotrope, which is difficult to separate by distillation, can be broken by the pervaporation. 3.3. Pervaporation of binary water/methanol mixtures Dehydration of alcohols is the system studied most extensively for pervaporation separation, and the dehydration of ethanol and isopropanol represents the best-developed pervaporation processes for commercial applications. However, almost all the membranes with reasonable selectivities for dehydration of isopropanol and ethanol exhibited much lower selectivity for methanol/water separation, primarily due to their smaller differences in the molecular sizes and affinities to the membranes. Though water and methanol do not form an azeotrope, their separation by distilla-
503
tion is still difficult because their relative volatility is small. It is of interest to test the efficiency of the chitosan membranes developed here for methanol/water separation by pervaporation, which is of also interest to DMC production. Fig. 7 illustrates the total flux and the separation factor for methanol dehydration as a function of operating temperature at different feed concentrations. In general, the membrane is more permeable to water than methanol, but the membrane exhibited a much lower permselectivity for methanol/water separation than for the separation of DMC/water and DMC/ methanol mixtures. The results in Fig. 7 show that the permeation flux increases as the operating temperature increases, whereas the separation factor decreases except when the feed methanol content is sufficiently high. To better illustrate the effect of feed composition on the pervaporation performance, the pervaporation data at a given temperature (45 ◦ C) were plotted versus feed methanol concentration, as shown in Fig. 8. It is shown that as the methanol content in the feed increases, both the partial fluxes of water and methanol decrease, and the separation factor
Fig. 9. Pervaporation data versus the vapor–liquid equilibrium data for the separation of methanol/water mixtures at 45 ◦ C.
504
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
gradually increases. However, at high methanol concentrations (>95 wt.%), a further increase in the methanol feed concentration increases the methanol flux rapidly, while the water flux continues to decrease slowly. Consequently, there exist a maximum separation factor and a minimum total flux at around 95 wt.% feed methanol concentration. Similar trend can also be observed for the pervaporation performance at other operating temperatures. The earlier mentioned results may be explained qualitatively. When the components in a water/methanol mixture permeate through the hydrophilic chitosan membrane, there are two important factors influencing the permeation process; one is the plasticization of the membrane matrix, and the other is the competition between the permeating species. Water is more hydrophilic and tends to swell the chitosan membrane. Membrane swelling results in a reduction in the membrane resistance to diffusion, which enhances
the permeation rates of both permeating components. Hence, at a higher water content in the feed, a higher flux and a lower selectivity are observed. On the other hand, when the water content in the feed is low enough, the membrane swelling becomes insignificant, rendering the membrane more discriminative to the permeation of different species. When the water content is very low, however, the methanol will dominate the overall permeation, making the dehydration of methanol less efficient. Fig. 9 compares the pervaporation data and the vapor–liquid equilibrium data for the separation of methanol/water mixtures. It shows that the permselectivity of the membrane studied here is not much higher than the separation factor that would be obtained in distillation at relatively high feed methanol concentrations that are of interest for methanol dehydration. However, pervaporation requires much lower energy consumption for the separation since
Table 2 Experimental data for pervaporation of ternary water/methanol/DMC mixtures Temperature (◦ C)
Flux (g/(m2 h))
Permeate concentration (wt.%) Water
Methanol
Total
DMC
Water
Methanol
DMC
Feed: water 1.1%, methanol 69.7%, DMC 29.2% 25 11.2 86.3 35 9.5 87.3 45 9.0 87.8 55 8.4 88.3
2.6 3.2 3.3 3.4
4.9 7.6 12.0 17.6
38.1 69.7 117.2 184.8
1.1 2.6 4.4 7.0
44.2 79.8 133.5 209.4
Feed: water 6.8%, methanol 65.4%, DMC 27.8% 25 24.2 73.0 35 26.7 70.2 45 27.6 69.1 55 28.0 68.8
2.8 3.1 3.4 3.2
31.3 49.1 71.6 102.0
94.5 129.0 179.3 250.2
3.6 5.7 8.8 11.7
129.4 183.8 259.6 363.9
Feed: water 0.1%, methanol 11.0%, DMC 88.9% 25 15.8 73.0 35 14.0 73.4 45 12.7 74.8 55 12.1 74.9
11.1 12.6 12.5 13.1
3.8 4.3 4.9 6.3
17.4 22.4 28.9 39.1
2.6 3.8 4.8 6.8
23.9 30.5 38.6 52.2
Feed: water 0.5%, methanol 10.0%, DMC 89.5% 29 43.3 48.2 35 43.1 48.5 45 43.0 49.8 55 42.9 51.0
8.5 8.4 7.3 6.2
9.9 10.0 16.6 22.8
11.0 11.3 19.3 27.1
1.9 1.9 2.8 3.3
22.8 23.2 38.7 53.2
Feed: water 1.1%, methanol 9.4%, DMC 89.5% 30 69.1 26.8 35 72.6 24.3 45 74.7 22.8 55 72.0 24.8
4.1 3.1 2.5 3.3
22.2 26.1 37.5 50.8
8.6 8.7 11.5 17.5
1.3 1.1 1.3 2.3
32.1 35.9 50.2 70.6
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
only the permeate stream undergoes evaporation, while in distillation the whole feed stream needs to undergo multiple stages of evaporation and condensation. From a DMC production point of view, even a moderate separation of water from methanol is still beneficial, especially when this can be accompanied in primary separation of DMC/methanol mixtures by pervaporation, as discussed later. 3.4. Pervaporation of ternary DMC/methanol/water mixtures The chitosan membrane was tested for the separation of ternary DMC/methanol/water mixtures. For convenience of comparing the experimental results, the composition of the ternary mixtures was kept at fixed methanol/DMC ratios with varying water content. Two general representative cases of the ternary mixtures pervaporation are investigated. One is the methanol-dominating mixtures for which a methanol/DMC ratio was kept at approximately 70/30 by weight, which is near the azeotropic composition,
505
a concentration similar to that of the typical top product stream from the methanol azeotrope column in the ENIChem process. The other is the DMC-dominating mixtures for which a methanol/DMC ratio of approximately 10/90 by weight was chosen, which represents the purification of DMC where methanol and water are the impurities to be removed. The experimental results on the pervaporation separation of the ternary mixtures are summarized in Table 2. It can be seen that pervaporation can be used to separate methanol and water from the ternary mixtures. As expected, the permeating species do not permeate independently, and there exist strong coupling effects among the three permeants. In general, the effect of operating temperature on the fluxes of the ternary mixtures is similar to that of binary methanol/DMC and water/methanol systems. In both the DMC- and the methanol-dominating ternary systems, an increase in the operating temperature leads to an increase in the partial fluxes of all the three components. Recall that for the binary water/DMC system, both water and DMC fluxes
Fig. 10. Total and partial permeation fluxes for separation of ternary DMC/methanol/water mixtures containing primarily 70/30 (by weight) methanol/DMC. Temperature, 45 ◦ C.
506
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
decrease with an increase in the temperature. This effect appears to have been suppressed overwhelmingly in the ternary systems by the strong methanol/DMC and water/methanol interactions, which appear to facilitate the permeation when the temperature increases. Based on the experimental data in Table 2, the following observations can further be made: (1) Methanol and water are enriched in the permeate, indicating that the membrane can be used to remove methanol and water from the ternary DMC/methanol/water mixtures. In addition, the enrichment of water is much more significant than methanol. (2) For the mixtures with a 70/30 methanol/DMC ratio, an increase in the feed water content from 1.1 to 6.8% does not affect the DMC concentration in the permeate significantly, whereas the water content in the permeate increases by a factor of 2–3 and that of methanol decreases by 15–22%, depending on the operating temperature.
(3) For the mixtures with a 10/90 methanol/DMC ratio, the effect of feed water concentration on the permeate composition is more significant. Depending on the temperature, an increase in the feed water content from 0.1 to 0.5% results in a reduction in the DMC content in the permeate by 23–50%, a reduction in methanol content by about 33%, and an increase in water content by a factor of 2.7–4.5. To further look at the interactions among the permeating components in the ternary system, the permeation fluxes at an operating temperature of 45 ◦ C are plotted as a function of water content in the feed, as shown in Figs. 10 and 11. The data indicate that the magnitude of the permeant–permeant interactions is affected by the feed composition. For both the 70/30 and the 10/90 methanol/DMC mixtures, the total permeation flux initially decreases as the feed water content increases and then the flux begins to increase when the feed water content is relatively low. While the partial flux of water is approximately
Fig. 11. Total and partial permeation fluxes for separation of ternary DMC/methanol/water mixtures containing primarily 90/10 (by weight) methanol/DMC. Temperature, 45 ◦ C.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
proportional to the water content in the feed over the experimental range of the feed water content tested, the partial fluxes of methanol and DMC are affected by the presence of water differently. For the 10/90 methanol/DMC mixtures, the presence of water slows down the permeation of both DMC and methanol, whereas no similar observations can be made for the 70/30 methanol/DMC mixtures. These results imply that the coupling effect is important to the pervaporation separation of the multi-component mixtures due to the interactions among the permeating components and the interactions between the permeating components and the membrane. It may be noted that homogeneous dense chitosan membranes were used in the present study. Because of its high hydrophilicity, thin-film composite chitosan membranes supported on a porous substrate are expected to result in a higher flux and stability. 4. Conclusions The separations of binary DMC/methanol, DMC/ water, methanol/water mixtures and ternary DMC/ methanol/water mixtures by pervaporation using chitosan membranes were studied. Chitosan membranes were prepared by casting an aqueous protonated chitosan solution, followed by alkaline treatment to convert chitosan in its free amine form. The effects of feed composition and operating temperature on the membrane performance were investigated, and the membrane properties under the experimental conditions that are of interest to the manufacturing of DMC were evaluated. It was demonstrated that the membrane could be used for DMC/methanol separation as well as dehydration of DMC, which represent an important task in the manufacturing of DMC. The test results on the ternary DMC/methanol/water mixture separation showed that the chitosan membrane was effective for separating off methanol and/or water from dimethyl carbonate, and the permeant–permeant interactions and the permeant–membrane interactions had a significant effect on the membrane performance. Acknowledgements Research support from the Natural Sciences and Engineering Research Council (NSERC) of Canada
507
and the Fielding Chemical Technologies Inc. (Mississauga, Ont.) is gratefully acknowledged.
References [1] Y. Ono, Catalysis in the production and reactions of dimethyl carbonate, an environmentally benign building block, Appl. Catal. A: Gen. 155 (1997) 133–166. [2] Environmental Protection Agency, Advance notice of intent to initiate rulemaking under the Toxic Substances Control Act to eliminate or limit the use of MTBE as a fuel additive in gasoline, Federal Register 65 (58) (2000) 16093–16109. [3] P.J. Gaylor, Modified fuel, US Patent no. 2,331,386 (1943). [4] M.A. Pacheco, C.L. Marshall, Review of dimethyl carbonate (DMC) manufacture and its characteristics as a fuel additive, Energy Fuels 11 (1997) 2–29. [5] W. Mehl, W. Scheinert, I. Janisch, A. Groschl, Process for separating off alkanols from other organic compounds of higher carbon number, US Patent no. 5,504,239 (1996). [6] H. Landscheidt, E. Wolters, P. Wagner, A. Klausener, Process for the preparation of dimethyl carbonate, US Patent no. 5,543,548 (1996). [7] A. Nickel, W. Arlt, I. Janisch, P. Wagner, A. Klausener, Process for separating off alkanols, mixtures of alkanols and water or water itself from oxygen-containing organic compounds of higher carbon number, US Patent no. 5,360,923 (1994). [8] M. Pasternak, C.R. Bartels, J. Reale Jr., Separation of organic liquids, US Patent no. 4,798,674 (1989). [9] V.M. Shah, C.R. Bartels, M. Pasternak, J. Reale Jr., Opportunities for membranes in the production of octane enhancers, AIChE Symp. Ser. 85 (272) (1989) 93–97. [10] R. Rautenbach, J. Vier, Design and analysis of combined distillation/pervaporation processes, in: R. Bakish (Ed.), Proceedings of Seventh International Conference on Pervaporation Processes in the Chem. Ind., Bakish Materials Corp., Englewood, NJ, 1995, pp. 70–85. [11] A. Tabe-Mohammadi, J.P.G. Villaluenca, H.J. Kim, T. Chan, V. Rauw, Effects of polymer solvents on the performance of cellulose acetate membranes in methanol/methyl tertiary butyl ether separation, J. Appl. Polym. Sci. 82 (2001) 2882–2895. [12] C. Bangxiao, Y. Li, Y. Hailin, G. Congjie, Effect of separating layer in pervaporation composite membrane for MTBE/MeOH separation, J. Membr. Sci. 194 (2001) 151– 156. [13] H.H. Schwarz, R. Apostel, D. Paul, Membranes based on polyelectrolyte-surfactant complexes for methanol separation, J. Membr. Sci. 194 (2001) 91–102. [14] M. Niang, G.S. Luo, A triacetate cellulose membrane for the separation of methyl tert-butyl ether/methanol mixtures by pervaporation, Sep. Purif. Technol. 4 (2001) 427–435. [15] M. Niang, G. Luo, P. Schaetzel, Pervaporation separation of methyl tert-butyl ether/methanol mixtures using a highperformance blended membrane, J. Appl. Polym. Sci. 64 (1997) 875–882.
508
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
[16] R.Y.M. Huang, G.Y. Moon, R. Pal, Chitosan/anionic surfactant complex membranes for the pervaporation separation of methanol/MTBE and characterization of the polymer/surfactant system, J. Membr. Sci. 184 (2001) 1–15. [17] M. Yoshikawa, T. Yoshioka, J. Fujime, A. Murakami, Pervaporation separation of MeOH/MTBE through agarose membranes, J. Membr. Sci. 178 (2000) 75–78. [18] S.G. Kim, G.T. Lim, J. Jegal, K.H. Lee, Pervaporation separation of MTBE (methyl tert-butyl ether) and methanol mixtures through polyion complex composite membranes consisting of sodium alginate/chitosan, J. Membr. Sci. 174 (2000) 1–15. [19] S.K. Ray, S.B. Sawant, V.G. Pangarkar, Development of methanol selective membranes for separation of methanol– methyl tertiary butyl ether mixtures by pervaporation, J. Appl. Polym. Sci. 74 (1999) 2645–2659. [20] S.G. Cao, Y.Q. Shi, G.W. Chen, Properties and pervaporation characteristics of chitosan–poly(N-vinyl-2-pyrrolidone) blend membranes for MeOH–MTBE, J. Appl. Polym. Sci. 74 (1999) 1452–1458. [21] J.S. Yang, H.J. Kim, W.H. Jo, Y.S. Kang, Analysis of pervaporation of methanol–MTBE mixtures through cellulose acetate and cellulose triacetate membranes, Polymer 39 (1998) 1381–1385.
[22] J.W. Rhim, Y.K. Kim, Pervaporation separation of MTBE– methanol mixtures using cross-linked PVA membranes, J. Appl. Polym. Sci. 75 (2000) 1699–1707. [23] H.C. Park, N.E. Ramaker, M.H.V. Mulder, C.A. Smolders, Separation of MTBE–methanol mixtures by pervaporation, Sep. Sci. Technol. 30 (1995) 419–433. [24] F. Doghieri, A. Nardella, G.C. Sarti, C. Valentini, Pervaporation of methanol–MTBE mixtures through modified poly(phenylene oxide) membranes, J. Membr. Sci. 91 (1994) 283–291. [25] M. Zhou, M. Persin, J. Sarrazin, Methanol removal from organic mixtures by pervaporation using polypyrrole membranes, J. Membr. Sci. 117 (1996) 303–309. [26] S. Nam, Y.M. Lee, Pervaporation separation of methanol/ methyl t-butyl ether through chitosan composite membrane modified with surfactants, J. Membr. Sci. 157 (1999) 63–71. [27] X. Feng, R.Y.M. Huang, Pervaporation with chitosan membranes. Part I. Separation of water from ethylene glycol by a chitosan/polysulfone composite membrane, J. Membr. Sci. 116 (1996) 67–76. [28] N.F. Giles, G.M. Wilson, Phase equilibria on seven binary mixtures, J. Chem. Eng. Data 45 (2000) 146–153. [29] L.H. Horsley, W.S. Tamplin, Azeotropic Data, American Chemical Society, Washington, DC, 1952.
Pervaporation with chitosan membranes: separation of dimethyl carbonate/methanol/water mixtures Wooyoung Won a , Xianshe Feng a,∗ , Darren Lawless b a
Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 b Fielding Chemical Technologies Inc., Mississauga, Ont., Canada L5C 1T7 Received 15 February 2002; received in revised form 20 May 2002; accepted 29 May 2002
Abstract This study deals with the separation of binary dimethyl carbonate (DMC)/methanol, DMC/water, and methanol/water mixtures as well as ternary DMC/methanol/water mixtures by pervaporation using chitosan membranes. It is relevant to the manufacturing of DMC, where the energy intensive extractive distillation or pressure swing distillation is used conventionally for the separation of the reaction mixtures. Chitosan membranes were prepared by solution casting, followed by alkaline treatment. The effects of feed composition and operating temperature on the separation performance were investigated, and the membrane properties under the experimental conditions that are of interest to the manufacturing of DMC were evaluated. It was demonstrated that the membrane exhibited good performance for the DMC/methanol separation as well as the dehydration of DMC. The membrane also showed the capability of dehydrating methanol, but with a lower permselectivity. For the separation of ternary DMC/methanol/water mixtures, the interactions among the permeating components were shown to have a significant effect on the membrane performance. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Chitosan membrane; Dimethyl carbonate; Methanol
1. Introduction Dimethyl carbonate (DMC) is an important chemical that is considered to be an environmentally benign building block. The application of dimethyl carbonate has been expanding rapidly in recent years. It has been used as a carbonylation agent to substitute the deadly phosgene for the manufacturing of polycarbonates and urethane polymers, and as an environmentally friendly substitute to replace dimethyl sulfate and methyl halides in methylation reactions [1]. Numerous studies have been done to evaluate the advantage ∗ Corresponding author. Fax: +1-519-746-4979. E-mail address: [email protected] (X. Feng).
of using dimethyl carbonate for other applications including, for example, as a low viscosity solvent in lithium ion battery, and as an oxygenate for internal combustion engine fuels. The Federal Reformulated Fuels Act of 2000 requires the US EPA to ban the use of methyl tert-butyl ether (MTBE) in gasoline due to environmental concerns [2]. Consequently, dimethyl carbonate has become a strong contender for replacing MTBE as an oxygenate in the unleaded gasoline to meet the Clean Air Act specifications of oxygen-laden additives in gasoline, and it is anticipated that the production of DMC will be increased substantially to meet the refinery industry’s potential need. Dimethyl carbonate has excellent characteristics as a fuel additive, including
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 3 6 7 - 8
494
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
high oxygen content (53 wt.%), good blending properties, favorable distribution in gasoline/water, low haze point, low toxicity, and fast biodegradation. Though DMC has not been used in gasoline commercially, the idea of using DMC in fuels is not new. In 1943, it was reported that DMC and other alkyl carbonates could reduce the surface tension of diesel fuels [3]. The beneficial fuel properties of DMC were disclosed in several patents in the 1980s; a chronological development on this account is described by Pacheco and Marshall [4], who also predicted that a substantially large scale-up of current DMC production would be needed to meet the potential market need. Separation is an important step in the DMC manufacturing. For example, in the ENIChem process, which is a major commercial process for DMC production based on oxidative carbonylation of methanol on a CuCl catalyst, the product stream comprises of a ternary DMC/methanol/water mixture. Removal of methanol, water or methanol/water from the mixture is especially important due to the azeotropic nature of the mixtures. Various separation processes (e.g. extractive distillation, pressure swing distillation, liquid–liquid extraction, adsorption on zeolites, and low temperature crystallization) have been proposed to separate and purify DMC [4]. Pervaporation separation, which is based on the selective permeation of components in a mixture through a membrane, appears to be a promising alternative. The idea of using pervaporation for separation of DMC/methanol mixtures was disclosed in a few patents [5–8]. Shah et al. [9] compared high-pressure distillation with a hybrid membrane/distillation system and concluded that the hybrid system could save both capital and operating costs. In the hybrid system, the pervaporation membrane unit removes methanol from the reactor effluent prior to distillation in order to suppress the formation of azeotrope. Rautenbach and Vier [10] carried out a simulation study for DMC/methanol separation with a hybrid membrane/distillation system; the feed was admitted to the distillation column or the pervaporation unit if it was at the azeotropic composition. It was shown that the hybrid system compares with the conventional two-pressure distillation favorably. In spite of the interest in separating DMC using pervaporation, very few studies are available in the
Table 1 Membranes used for methanol/MTBE separation by pervaporation Membrane
Reference
Chitosan Polyvinyl alcohol Cellulose esters Poly(phenylene oxide) Polypyrrole Polyelectrolyte Agarose Acrylonitrile copolymer
[16,18,20,26] [12,22,23] [11,14,15,21] [24] [25] [13] [17] [19]
literature that address the preparation and performance of pervaporation membranes for DMC separation. This is presumably due to the fact that MTBE has been an authorized oxygenate in gasoline since the late 1970s and that MTBE have been the dominating cost-effective oxygenate ever since. In fact, the MTBE/methanol separation, which is relevant to MTBE manufacturing, by pervaporation has been investigated extensively, as illustrated in Table 1, which summarizes the recent work on membranes used for this separation. Considering the physicochemical similarities of MTBE and DMC, the prior work on pervaporation separation of MTBE/methanol mixtures suggests that chitosan- and polyvinyl alcohol-based polymers may be good candidate materials for preparing the pervaporation membranes for DMC/methanol separation. In the present study, the pervaporation separation of binary DMC/methanol, DMC/water, methanol/water mixtures and ternary DMC/methanol/water mixtures by chitosan membranes were studied. Chitosan is a polysaccharide that is insoluble in water at neutral pH’s. It is a linear polymer comprised primarily of glucosamine [27]. Chitosan was chosen as the membrane material because of its good film forming property and chemical stability as well as its demonstrated favorable performance for MTBE/methanol separation. The membranes were prepared by casting aqueous protonated chitosan solution, followed by alkaline treatment to convert chitosan in its free amine form. The effects of feed composition and temperature on the membrane performance were investigated, and the membrane properties under the experimental conditions that are of interest to the manufacturing of DMC were evaluated.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
495
2. Experimental 2.1. Materials Chitosan (Flonac N, molecular weight 100,000) in flake form was supplied by Kyowa Technos Co. Ltd., Japan. Methanol, ethanol and acetic acid were purchased from BDH Chemical, and dimethyl carbonate was purchased from Aldrich Chemical Company. All the organic solvents used in the experiments were of regent grade. De-ionized laboratory water was used in making the aqueous solutions for membrane preparation and treatment. The feed mixtures used for the pervaporation experiments were prepared by blending methanol, dimethyl carbonate and/or water with a predetermined composition. 2.2. Membrane preparation Homogeneous dense chitosan membranes were prepared by the solution casting technique. Firstly, chitosan was dissolved in a dilute aqueous acetic acid solution where the amine groups of chitosan were protonated by the acid to form a homogeneous solution. The polymer solution, comprising of 1.1 wt.% chitosan, 2 wt.% acetic acid, and 96.9 wt.% water, was filtered to remove a trace amount of non-dissolved residual solids. The chitosan solution was then cast onto a horizontally positioned glass plate with a Gardner knife at ambient conditions. The cast chitosan solution film was consequently air-dried in an environmentally-controlled chamber, which was supplied by D.F.S. Inc., France. The air-dried chitosan membrane, which was in the form of chitosan salt, was subject to an alkaline treatment so that the cationic amine groups (–NH3 + ) in chitosan were converted into the free amine form (–NH2 ). The alkaline solution contained 0.8 M sodium hydroxide in a 50/50 (v/v) ethanol/water solution. After alkaline treatment for 24 h, the membrane was thoroughly washed and rinsed with de-ionized water, and finally dried in ambient air. The thickness of the resulting membrane was 28.4 m. 2.3. Pervaporation The apparatus used for pervaporation experiments is shown schematically in Fig. 1. The membrane was
Fig. 1. Schematic diagram of pervaporation setup: (1) thermostat control; (2) feed tank; (3) heating device; (4) feed circulation pump; (5) pervaporation cell; (6) cold trap; (7) vacuum pump.
housed in a pervaporation cell that consisted of two detachable stainless steel parts. A porous steel plate was imbedded in one of the parts to support the membrane, and the two parts were set in proper alignment. Rubber O-rings were used to provide a pressure tight seal between the membrane and the permeation cell. The effective area of permeation was 14.2 cm2 . The feed mixture with a predetermined composition was circulated from a thermostated feed tank to the permeation cell using a feed pump, and the retentate from the permeation cell was recycled to the feed tank. Vacuum was applied to the permeate side of the membrane, and the permeate vapor was condensed and collected in a Pyrex glass cold trap immersed in liquid nitrogen. In all the experiments, the feed was kept at atmospheric pressure, whereas the permeate pressure was maintained at 2–3 mbar, which was monitored by a Pirani vacuum gauge (MKS Instruments). The permeation rate was determined gravimetrically
496
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
by weighing the permeate sample collected over a given period of time using an analytical balance, and the permeate composition was analyzed using a Hewlett-Packard gas chromatography (HP5890 Series II) equipped with a thermal conductivity detector. The feed circulation rate was kept substantially high as compared to the permeation rate so as to minimize the effect of concentration polarization. Though the pervaporation was carried out batchwise, the amount of permeate removed by the membrane was kept below 0.1% of the initial feed loading so that the feed concentration was essentially constant during pervaporation runs. This study was concerned
with steady state permeation. All the experimental data were collected after steady state of pervaporation had been reached. The time required to reach the steady state was between 1 and 4 h, depending on the permeation flux through the membrane. The membrane was tested for the separations of binary DMC/methanol, DMC/water and methanol/water mixtures as well as ternary DMC/methanol/water mixtures. The permeate concentration and the permeation flux were used to characterize the membrane performance. In some cases involving the permeation of binary mixtures, separation factor was also used to characterize the permselectivity of the membrane.
Fig. 2. Total permeation flux and water concentration in permeate for separation of water from dimethyl carbonate.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
3. Results and discussions Dimethyl carbonate and water are partially miscible. At 20 ◦ C, the aqueous DMC solution undergoes a phase separation when the water content is in the range of 2.7–87 wt.% [28]. Typically, the product stream from a DMC reactor, consisting of 50–70% methanol, 30–40% DMC and 2–5% water, is subject to distillation where the DMC/methanol azeotrope from the top of the distillation tower is recycled back to the reactor, whereas the water “wet” DMC is withdrawn from the bottom. The wet DMC is then cooled to induce phase separation, and both
497
phases can be further distilled until the DMC/water azeotropic composition is approached. The DMC product so obtained may still contain a small amount of water. Obviously, from a separation point of view, it is of interest to investigate pervaporation separation of the binary and ternary mixtures, especially when (i) the composition is close to the azeotropic point, or (ii) water is the minor impurity in the DMC product that needs to be removed. This study was primarily focused on dehydrating DMC, de-bottlenecking DMC/methanol azeotrope, and the removal of water/methanol from ternary DMC/methanol/water mixtures.
Fig. 3. Partial permeation fluxes of dimethyl carbonate and water for the dehydration of dimethyl carbonate.
498
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
3.1. Pervaporation of binary DMC/water mixtures The concentration range for homogeneous binary DMC/water solution is narrow, and caution was exercised in the measurement of permeate composition in case there was a phase separation in the permeate. Three concentration levels were chosen for the DMC dehydration study: 97.4, 98, and 99% of DMC by weight. The operating temperature was varied from 25 to 65 ◦ C for each concentration level. Over this experimental range explored, the permeate water concentration was found to be over 95 wt.%, within the homogeneous region of the DMC/water mixtures. Fig. 2 shows the total permeation flux and the perme-
ate concentration at different feed concentrations and operating temperatures. It can be seen that the water concentration in the permeate tends to be higher at a higher content of feed water. At a water content of 2.6% in feed, the permeate water content of greater than 97.5% was obtained in the temperature range tested, which corresponds to a separation factor of approximately 1460. Similarly, at a feed water content of 2%, the permeate water concentration was shown to be over 95%, corresponding to a separation factor of 930. Over the entire temperature range tested, the concentration of the permeate did not change significantly, which implies that the membrane is thermally stable for this system. The data in Fig. 2 also showed that the
Fig. 4. Pervaporation performance for the separation of binary dimethyl carbonate/methanol mixtures at different feed concentrations and operating temperatures.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
higher the feed water content is, the more significantly the permeation flux is affected by the temperature. Nickel et al. [7] reported that at a water content of 2.7% in the feed, a permeation flux of 0.8 kg/(m2 h) and a separation factor of 644 were obtained with the polyvinyl alcohol/polyacrylonitrile composite membrane developed by GFT at a temperature that was presumably between 50 and 70 ◦ C. Apparently, the chitosan membrane studied here is more selective to the DMC/water permeation, and the relatively low flux
499
of the membrane is due to the fact that the membrane used for this study was a thick dense homogeneous membrane. The permeation flux can be increased by the use of asymmetric composite membranes where the effective thickness of the membrane is reduced. It is interesting to note that unlike many other pervaporation systems where an increase in temperature generally leads to an increase in the permeation flux, the chitosan membrane showed a negative temperature dependence of the total permeation flux.
Fig. 5. Partial permeation fluxes of dimethyl carbonate and methanol for DMC/methanol separations.
500
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
This can be explained in the following. Based on the solution–diffusion model, the sorption and diffusion are the two major steps in pervaporation transport that control the permeation. In many cases, the diffusion is the rate-controlling step, and the effect of diffusion on the total flux is more significant than that of preferential sorption. The characteristics that distinguishes the permeation behavior of the DMC/water system from others is the poor miscibility of water with DMC. As the temperature increases, the solubility of water in DMC increases. As a result, at a given concentration, the activity of water in the homogeneous DMC/water solution decreases since more water can be dissolved by the DMC. In other words, the forces that trap water in the solution become stronger, which results in less water being sorbed into the membrane matrix, thereby slowing down the diffusion rate of water because of the reduced driving force across the membrane. In addition, the reduced sorption of water in the membrane matrix due to temperature increase will also reduce the plasticization of the membrane, and consequently the membrane displays a stronger resistance
to diffusion of both components. This hypothesis is in agreement with the experimental observations that increasing the temperature reduces both the partial fluxes of DMC and water, as shown in Fig. 3, which also implies that the plasticization of the membrane by water significantly affects the DMC permeation. At a higher water content in the feed, the permeation flux of DMC tends to be higher. It may be pointed out that DMC and water form an azeotrope at 77.5 ◦ C with a composition of 11% water and 89% DMC [29]. As such, from a practical application point of view, such an azeotrope can easily be broken by simply lowering the temperature to induce phase separation. Therefore, the pervaporation separation was not targeted to this azeotrope mixture in the present study. On the other hand, the membrane preferentially permeates water, and it is not anticipated to be effective to preferentially remove small amounts of DMC dissolved in water where water is the major component. A hydrophobic membrane that preferentially permeates DMC would be preferred for such an application.
Fig. 6. Permeate concentration as a function of feed concentration for pervaporation separation as compared to the vapor–liquid equilibrium (VLE) data.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
3.2. Pervaporation of binary DMC/methanol mixtures Fig. 4 shows the performance of the chitosan membrane for the separation of binary DMC/methanol mixtures at different operating temperatures. The membrane is preferentially permeable to methanol, and at a given temperature, as the methanol content in the feed increases, the overall permeation flux increases whereas the separation factor decreases. Unlike the aforementioned water/DMC separation, an increase in the operating temperature increases the overall permeation rate, and the separation factor, however, tends to decrease. Similar results have been observed for other pervaporation systems that
501
separate the MTBE/methanol mixtures [22,25,26]. A closer look at the partial fluxes, which are shown in Fig. 5, shows that both partial fluxes of DMC and methanol increase as the temperature increases, but the increase in the partial flux of DMC is generally more significant than that of methanol, leading to a gradual decrease in the separation factor. It may be pointed out that at a given operating temperature, the partial flux of methanol, which permeates through the membrane preferentially over DMC, tends to increase as the feed methanol concentration increases, whereas no such an obvious trend can be observed for the DMC permeation. It appears that at relatively low DMC concentrations, the DMC flux increases as
Fig. 7. Total permeation flux and separation factor for methanol dehydration at different operating temperatures and feed concentrations.
502
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
the DMC concentration increases and the opposite holds when the DMC concentration is sufficiently high. The experimental data of Nickel et al. [7] for DMC/methanol separation using a GFT membrane showed similar trend. Pasternak et al. [8] tested polyvinyl alcohol membranes for the separation of a methanol/DMC mixture that was close to the azeotropic composition (comprising of approximately 70/30 methanol/DMC by weight), and a permeate concentration of 93–97 wt.%
methanol and a total flux of 100–1130 g/(m2 h) were obtained, depending on the curing conditions of the membrane. This is comparable to the results obtained in this study. Fig. 6 shows the permeate concentration as a function of feed concentration as compared to the vapor–liquid equilibrium (VLE) data; for comparison, the pervaporation data of Rautenbach and Vier [10] using the GFT membrane at 70 ◦ C are also shown. The chitosan membrane appears to be more selective than the GFT membrane, which was
Fig. 8. Illustration of the effect of feed composition on the pervaporation performance of methanol/water separation at 45 ◦ C.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
primarily developed for dehydration of alcohols. It is clear that the concentration of methanol in the permeate is much higher than that in the saturated vapor over the entire concentration range. These results indicate that pervaporation is more efficient for DMC/methanol separation than distillation, and that the azeotrope, which is difficult to separate by distillation, can be broken by the pervaporation. 3.3. Pervaporation of binary water/methanol mixtures Dehydration of alcohols is the system studied most extensively for pervaporation separation, and the dehydration of ethanol and isopropanol represents the best-developed pervaporation processes for commercial applications. However, almost all the membranes with reasonable selectivities for dehydration of isopropanol and ethanol exhibited much lower selectivity for methanol/water separation, primarily due to their smaller differences in the molecular sizes and affinities to the membranes. Though water and methanol do not form an azeotrope, their separation by distilla-
503
tion is still difficult because their relative volatility is small. It is of interest to test the efficiency of the chitosan membranes developed here for methanol/water separation by pervaporation, which is of also interest to DMC production. Fig. 7 illustrates the total flux and the separation factor for methanol dehydration as a function of operating temperature at different feed concentrations. In general, the membrane is more permeable to water than methanol, but the membrane exhibited a much lower permselectivity for methanol/water separation than for the separation of DMC/water and DMC/ methanol mixtures. The results in Fig. 7 show that the permeation flux increases as the operating temperature increases, whereas the separation factor decreases except when the feed methanol content is sufficiently high. To better illustrate the effect of feed composition on the pervaporation performance, the pervaporation data at a given temperature (45 ◦ C) were plotted versus feed methanol concentration, as shown in Fig. 8. It is shown that as the methanol content in the feed increases, both the partial fluxes of water and methanol decrease, and the separation factor
Fig. 9. Pervaporation data versus the vapor–liquid equilibrium data for the separation of methanol/water mixtures at 45 ◦ C.
504
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
gradually increases. However, at high methanol concentrations (>95 wt.%), a further increase in the methanol feed concentration increases the methanol flux rapidly, while the water flux continues to decrease slowly. Consequently, there exist a maximum separation factor and a minimum total flux at around 95 wt.% feed methanol concentration. Similar trend can also be observed for the pervaporation performance at other operating temperatures. The earlier mentioned results may be explained qualitatively. When the components in a water/methanol mixture permeate through the hydrophilic chitosan membrane, there are two important factors influencing the permeation process; one is the plasticization of the membrane matrix, and the other is the competition between the permeating species. Water is more hydrophilic and tends to swell the chitosan membrane. Membrane swelling results in a reduction in the membrane resistance to diffusion, which enhances
the permeation rates of both permeating components. Hence, at a higher water content in the feed, a higher flux and a lower selectivity are observed. On the other hand, when the water content in the feed is low enough, the membrane swelling becomes insignificant, rendering the membrane more discriminative to the permeation of different species. When the water content is very low, however, the methanol will dominate the overall permeation, making the dehydration of methanol less efficient. Fig. 9 compares the pervaporation data and the vapor–liquid equilibrium data for the separation of methanol/water mixtures. It shows that the permselectivity of the membrane studied here is not much higher than the separation factor that would be obtained in distillation at relatively high feed methanol concentrations that are of interest for methanol dehydration. However, pervaporation requires much lower energy consumption for the separation since
Table 2 Experimental data for pervaporation of ternary water/methanol/DMC mixtures Temperature (◦ C)
Flux (g/(m2 h))
Permeate concentration (wt.%) Water
Methanol
Total
DMC
Water
Methanol
DMC
Feed: water 1.1%, methanol 69.7%, DMC 29.2% 25 11.2 86.3 35 9.5 87.3 45 9.0 87.8 55 8.4 88.3
2.6 3.2 3.3 3.4
4.9 7.6 12.0 17.6
38.1 69.7 117.2 184.8
1.1 2.6 4.4 7.0
44.2 79.8 133.5 209.4
Feed: water 6.8%, methanol 65.4%, DMC 27.8% 25 24.2 73.0 35 26.7 70.2 45 27.6 69.1 55 28.0 68.8
2.8 3.1 3.4 3.2
31.3 49.1 71.6 102.0
94.5 129.0 179.3 250.2
3.6 5.7 8.8 11.7
129.4 183.8 259.6 363.9
Feed: water 0.1%, methanol 11.0%, DMC 88.9% 25 15.8 73.0 35 14.0 73.4 45 12.7 74.8 55 12.1 74.9
11.1 12.6 12.5 13.1
3.8 4.3 4.9 6.3
17.4 22.4 28.9 39.1
2.6 3.8 4.8 6.8
23.9 30.5 38.6 52.2
Feed: water 0.5%, methanol 10.0%, DMC 89.5% 29 43.3 48.2 35 43.1 48.5 45 43.0 49.8 55 42.9 51.0
8.5 8.4 7.3 6.2
9.9 10.0 16.6 22.8
11.0 11.3 19.3 27.1
1.9 1.9 2.8 3.3
22.8 23.2 38.7 53.2
Feed: water 1.1%, methanol 9.4%, DMC 89.5% 30 69.1 26.8 35 72.6 24.3 45 74.7 22.8 55 72.0 24.8
4.1 3.1 2.5 3.3
22.2 26.1 37.5 50.8
8.6 8.7 11.5 17.5
1.3 1.1 1.3 2.3
32.1 35.9 50.2 70.6
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
only the permeate stream undergoes evaporation, while in distillation the whole feed stream needs to undergo multiple stages of evaporation and condensation. From a DMC production point of view, even a moderate separation of water from methanol is still beneficial, especially when this can be accompanied in primary separation of DMC/methanol mixtures by pervaporation, as discussed later. 3.4. Pervaporation of ternary DMC/methanol/water mixtures The chitosan membrane was tested for the separation of ternary DMC/methanol/water mixtures. For convenience of comparing the experimental results, the composition of the ternary mixtures was kept at fixed methanol/DMC ratios with varying water content. Two general representative cases of the ternary mixtures pervaporation are investigated. One is the methanol-dominating mixtures for which a methanol/DMC ratio was kept at approximately 70/30 by weight, which is near the azeotropic composition,
505
a concentration similar to that of the typical top product stream from the methanol azeotrope column in the ENIChem process. The other is the DMC-dominating mixtures for which a methanol/DMC ratio of approximately 10/90 by weight was chosen, which represents the purification of DMC where methanol and water are the impurities to be removed. The experimental results on the pervaporation separation of the ternary mixtures are summarized in Table 2. It can be seen that pervaporation can be used to separate methanol and water from the ternary mixtures. As expected, the permeating species do not permeate independently, and there exist strong coupling effects among the three permeants. In general, the effect of operating temperature on the fluxes of the ternary mixtures is similar to that of binary methanol/DMC and water/methanol systems. In both the DMC- and the methanol-dominating ternary systems, an increase in the operating temperature leads to an increase in the partial fluxes of all the three components. Recall that for the binary water/DMC system, both water and DMC fluxes
Fig. 10. Total and partial permeation fluxes for separation of ternary DMC/methanol/water mixtures containing primarily 70/30 (by weight) methanol/DMC. Temperature, 45 ◦ C.
506
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
decrease with an increase in the temperature. This effect appears to have been suppressed overwhelmingly in the ternary systems by the strong methanol/DMC and water/methanol interactions, which appear to facilitate the permeation when the temperature increases. Based on the experimental data in Table 2, the following observations can further be made: (1) Methanol and water are enriched in the permeate, indicating that the membrane can be used to remove methanol and water from the ternary DMC/methanol/water mixtures. In addition, the enrichment of water is much more significant than methanol. (2) For the mixtures with a 70/30 methanol/DMC ratio, an increase in the feed water content from 1.1 to 6.8% does not affect the DMC concentration in the permeate significantly, whereas the water content in the permeate increases by a factor of 2–3 and that of methanol decreases by 15–22%, depending on the operating temperature.
(3) For the mixtures with a 10/90 methanol/DMC ratio, the effect of feed water concentration on the permeate composition is more significant. Depending on the temperature, an increase in the feed water content from 0.1 to 0.5% results in a reduction in the DMC content in the permeate by 23–50%, a reduction in methanol content by about 33%, and an increase in water content by a factor of 2.7–4.5. To further look at the interactions among the permeating components in the ternary system, the permeation fluxes at an operating temperature of 45 ◦ C are plotted as a function of water content in the feed, as shown in Figs. 10 and 11. The data indicate that the magnitude of the permeant–permeant interactions is affected by the feed composition. For both the 70/30 and the 10/90 methanol/DMC mixtures, the total permeation flux initially decreases as the feed water content increases and then the flux begins to increase when the feed water content is relatively low. While the partial flux of water is approximately
Fig. 11. Total and partial permeation fluxes for separation of ternary DMC/methanol/water mixtures containing primarily 90/10 (by weight) methanol/DMC. Temperature, 45 ◦ C.
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
proportional to the water content in the feed over the experimental range of the feed water content tested, the partial fluxes of methanol and DMC are affected by the presence of water differently. For the 10/90 methanol/DMC mixtures, the presence of water slows down the permeation of both DMC and methanol, whereas no similar observations can be made for the 70/30 methanol/DMC mixtures. These results imply that the coupling effect is important to the pervaporation separation of the multi-component mixtures due to the interactions among the permeating components and the interactions between the permeating components and the membrane. It may be noted that homogeneous dense chitosan membranes were used in the present study. Because of its high hydrophilicity, thin-film composite chitosan membranes supported on a porous substrate are expected to result in a higher flux and stability. 4. Conclusions The separations of binary DMC/methanol, DMC/ water, methanol/water mixtures and ternary DMC/ methanol/water mixtures by pervaporation using chitosan membranes were studied. Chitosan membranes were prepared by casting an aqueous protonated chitosan solution, followed by alkaline treatment to convert chitosan in its free amine form. The effects of feed composition and operating temperature on the membrane performance were investigated, and the membrane properties under the experimental conditions that are of interest to the manufacturing of DMC were evaluated. It was demonstrated that the membrane could be used for DMC/methanol separation as well as dehydration of DMC, which represent an important task in the manufacturing of DMC. The test results on the ternary DMC/methanol/water mixture separation showed that the chitosan membrane was effective for separating off methanol and/or water from dimethyl carbonate, and the permeant–permeant interactions and the permeant–membrane interactions had a significant effect on the membrane performance. Acknowledgements Research support from the Natural Sciences and Engineering Research Council (NSERC) of Canada
507
and the Fielding Chemical Technologies Inc. (Mississauga, Ont.) is gratefully acknowledged.
References [1] Y. Ono, Catalysis in the production and reactions of dimethyl carbonate, an environmentally benign building block, Appl. Catal. A: Gen. 155 (1997) 133–166. [2] Environmental Protection Agency, Advance notice of intent to initiate rulemaking under the Toxic Substances Control Act to eliminate or limit the use of MTBE as a fuel additive in gasoline, Federal Register 65 (58) (2000) 16093–16109. [3] P.J. Gaylor, Modified fuel, US Patent no. 2,331,386 (1943). [4] M.A. Pacheco, C.L. Marshall, Review of dimethyl carbonate (DMC) manufacture and its characteristics as a fuel additive, Energy Fuels 11 (1997) 2–29. [5] W. Mehl, W. Scheinert, I. Janisch, A. Groschl, Process for separating off alkanols from other organic compounds of higher carbon number, US Patent no. 5,504,239 (1996). [6] H. Landscheidt, E. Wolters, P. Wagner, A. Klausener, Process for the preparation of dimethyl carbonate, US Patent no. 5,543,548 (1996). [7] A. Nickel, W. Arlt, I. Janisch, P. Wagner, A. Klausener, Process for separating off alkanols, mixtures of alkanols and water or water itself from oxygen-containing organic compounds of higher carbon number, US Patent no. 5,360,923 (1994). [8] M. Pasternak, C.R. Bartels, J. Reale Jr., Separation of organic liquids, US Patent no. 4,798,674 (1989). [9] V.M. Shah, C.R. Bartels, M. Pasternak, J. Reale Jr., Opportunities for membranes in the production of octane enhancers, AIChE Symp. Ser. 85 (272) (1989) 93–97. [10] R. Rautenbach, J. Vier, Design and analysis of combined distillation/pervaporation processes, in: R. Bakish (Ed.), Proceedings of Seventh International Conference on Pervaporation Processes in the Chem. Ind., Bakish Materials Corp., Englewood, NJ, 1995, pp. 70–85. [11] A. Tabe-Mohammadi, J.P.G. Villaluenca, H.J. Kim, T. Chan, V. Rauw, Effects of polymer solvents on the performance of cellulose acetate membranes in methanol/methyl tertiary butyl ether separation, J. Appl. Polym. Sci. 82 (2001) 2882–2895. [12] C. Bangxiao, Y. Li, Y. Hailin, G. Congjie, Effect of separating layer in pervaporation composite membrane for MTBE/MeOH separation, J. Membr. Sci. 194 (2001) 151– 156. [13] H.H. Schwarz, R. Apostel, D. Paul, Membranes based on polyelectrolyte-surfactant complexes for methanol separation, J. Membr. Sci. 194 (2001) 91–102. [14] M. Niang, G.S. Luo, A triacetate cellulose membrane for the separation of methyl tert-butyl ether/methanol mixtures by pervaporation, Sep. Purif. Technol. 4 (2001) 427–435. [15] M. Niang, G. Luo, P. Schaetzel, Pervaporation separation of methyl tert-butyl ether/methanol mixtures using a highperformance blended membrane, J. Appl. Polym. Sci. 64 (1997) 875–882.
508
W. Won et al. / Journal of Membrane Science 209 (2002) 493–508
[16] R.Y.M. Huang, G.Y. Moon, R. Pal, Chitosan/anionic surfactant complex membranes for the pervaporation separation of methanol/MTBE and characterization of the polymer/surfactant system, J. Membr. Sci. 184 (2001) 1–15. [17] M. Yoshikawa, T. Yoshioka, J. Fujime, A. Murakami, Pervaporation separation of MeOH/MTBE through agarose membranes, J. Membr. Sci. 178 (2000) 75–78. [18] S.G. Kim, G.T. Lim, J. Jegal, K.H. Lee, Pervaporation separation of MTBE (methyl tert-butyl ether) and methanol mixtures through polyion complex composite membranes consisting of sodium alginate/chitosan, J. Membr. Sci. 174 (2000) 1–15. [19] S.K. Ray, S.B. Sawant, V.G. Pangarkar, Development of methanol selective membranes for separation of methanol– methyl tertiary butyl ether mixtures by pervaporation, J. Appl. Polym. Sci. 74 (1999) 2645–2659. [20] S.G. Cao, Y.Q. Shi, G.W. Chen, Properties and pervaporation characteristics of chitosan–poly(N-vinyl-2-pyrrolidone) blend membranes for MeOH–MTBE, J. Appl. Polym. Sci. 74 (1999) 1452–1458. [21] J.S. Yang, H.J. Kim, W.H. Jo, Y.S. Kang, Analysis of pervaporation of methanol–MTBE mixtures through cellulose acetate and cellulose triacetate membranes, Polymer 39 (1998) 1381–1385.
[22] J.W. Rhim, Y.K. Kim, Pervaporation separation of MTBE– methanol mixtures using cross-linked PVA membranes, J. Appl. Polym. Sci. 75 (2000) 1699–1707. [23] H.C. Park, N.E. Ramaker, M.H.V. Mulder, C.A. Smolders, Separation of MTBE–methanol mixtures by pervaporation, Sep. Sci. Technol. 30 (1995) 419–433. [24] F. Doghieri, A. Nardella, G.C. Sarti, C. Valentini, Pervaporation of methanol–MTBE mixtures through modified poly(phenylene oxide) membranes, J. Membr. Sci. 91 (1994) 283–291. [25] M. Zhou, M. Persin, J. Sarrazin, Methanol removal from organic mixtures by pervaporation using polypyrrole membranes, J. Membr. Sci. 117 (1996) 303–309. [26] S. Nam, Y.M. Lee, Pervaporation separation of methanol/ methyl t-butyl ether through chitosan composite membrane modified with surfactants, J. Membr. Sci. 157 (1999) 63–71. [27] X. Feng, R.Y.M. Huang, Pervaporation with chitosan membranes. Part I. Separation of water from ethylene glycol by a chitosan/polysulfone composite membrane, J. Membr. Sci. 116 (1996) 67–76. [28] N.F. Giles, G.M. Wilson, Phase equilibria on seven binary mixtures, J. Chem. Eng. Data 45 (2000) 146–153. [29] L.H. Horsley, W.S. Tamplin, Azeotropic Data, American Chemical Society, Washington, DC, 1952.